Surface hardened composite metal article of manufacture



J. L. ELLIS ETAL Feb. 13, 1968 SURFACE HARDENED COMPOSITE METAL ARTICLE OF MANUFACTURE Filed April 6, 1965 FIG. 3

INVENTORS. JOHN L. ELL/S BY STUART E. TARKAN ATTORNE).

FIGS

3 368,882 SURFA'CE HARDEDIED COMPOSITE METAL ARTICLE OF MANUFACTURE John L. Ellis, White Plains, and Stuart E. Tarkan,

N.Y., assignors to Chromalloy a corporation of New York Filed Apr. 6, 1965, Ser. No. 445,917 3 Claims. (Cl. 29-195) Monsey, American Corporation,

ABSTRACT OF THE DISCLOSURE This invention relates to a composite metal article of manufacture comprising primary grains of a refractory compound material distributed through a matrix metal and, in particular, to a composite metal article comprising primary grains of a refractory metal carbide embedded in a steel matrix, with said carbide grains dispersed throughout the matrix and with at least the surface of the exposed matrix metal having a hard nitride layer or coating surrounding the primary grains.

It is known to produce sintered composite metal articles from a powdered mixture of refractory compound material, such as primary grains of a refractory metal carbide, and metal powder by compressing the powder mixture in a mold to produce a desired shape and sintering the shape at an elevated temperature in a substantially non-oxidizing atmosphere, whereby to cause the compressed particles to sinter together into a dense structure. In the case of refractory carbide hard metals consisting in large part of carbides of tungsten, titanium, and/ or other refractory carbides cemented together by liquid phase sintering in the matrix metal, the favorable properties are due in large part to the rather high intrinsic hardness of the carbides combined with the strengthening effect of the bonding metal. By the term primary carbide grains is meant the grains or particles of the refractory carbide added directly to the composition during the formulation thereof, and which still retain substantially their identity in the final composition as compared to secondary carbide grains which form by reaction during heat treatment.

In recent years, a machinable refractory carbide body comprising primary titanium carbide grains dispersed through a heat-treatable steel matrix has been developed which utilizes the intrinsic hardening effect of the primary carbide combined with the hardenability of the steel matrix. The machinable carbide body has one advantage over conventional cemented carbides in that the matrix of a composition containing about 50% by volume TiC and the balance substantially steel can be softened by annealing so as to lower the gross hardness of the composition to say 4OR such that the body can be machined to a desired shape and then hardened to upwards of 72R by quenching the alloy from an elevated temperature similarly as is done in certain of the alloy tool steels.

In producing refractory carbide compositions of the foregoing type with heat treatable or non-heat treatable steel matrices, refractory carbide powder, such as titanium carbide, is mixed with finely divided steel-forming ingrenited States Patent III clients, followed by compacting the mixture into the desired shape in a mold, and then subjecting the resulting compact to liquid phase sintering by heating it to a temperature above the lowest melting phase of the matrix metal but below the melting point of the refractory carbide. We find liquid phase sintering advantageous for our purposes in that dense products substantially free from porosity are obtainable, The foregoing type of composite metal has found use in a wide variety of applications such as forming dies, die nibs, wear resisting parts, size gages, machine parts, and the like.

In utilizing such refractory carbide compositions in the fabrication of a wide variety of tools or machine parts, it is sometimes desirable to modify the metallurgical characteristics of the matrix metal in order to meet the particular requirements stipulated by a specific end use intended for the tool. For example, in the case where a composition comprising 45% by volume of TiC dispersed through a stainless steel matrix making up 55% by volume of the composition is employed as a part in a pump involving sliding friction at an elevated tempertaure, a preferential wearing is apt to occur in the matrix metal between the carbide grains. This composition has an average hardness in the neighborhood of about 45R Since stainless steel of the 18/8 variety is substantially non-heat treatable, the intrinsic hardness of the steel is substantially below that of the actual hardness of the carbide. Therefore, when a machine part of the foregoing material is subjected to sliding friction at an elevated temperature, the soft matrix metal is preferentially worn away from around the carbide grains. While the primary carbide grains themselves provide resistance to wearing, they only do so provided they are securely embedded and anchored in the matrix metal. However, as the matrix metal is selectively worn or eroded away from around the grains, the grains tend to loosen and fall out. Such preferential wearing has its disadvantages in that it may result in a surface notch effect which lowers the resistance of the metal to impact. Thus, it would be desirable where the matrix is concerned to provide high retained hardness at the surface of the machine part to avoid the aforementioned difficulties.

In the case of a heat treatable composition, for example, a composition comprising about 50% by volume of TiC and the "balance 50% by volume of a low chrome low moly steel, it may be desirable to temper the matrix to substantially below the heat treated hardness where toughness throughout the cross section is an important criterion. For example, it may be desirable to temper the foregoing composition from a hardness of about R to a hardness of about 50 to SSR let us say, in the case of a draw die. However, it would be as important to insure a hard wear-resisting surface at the throat or orifice of the die, particularly between the hard primary carbide grains embedded in the matrix. Thus, it would be desirable to provide a hardened surface between the carbide grains to anchor said grains in place so as to obviate any preferential wearing away of the matrix and inhibit dislodgement of carbide particles.

The production of thread gages from hardened refractory carbide compositions of the foregoing type presents the problem of grinding costs in that the form grinding wheel employed in producing the threads tends to break down rapidly. On the other hand, if a composition containing 50% by volume of TiC and 50% by volume of a heat treatable steel, such as a low chrome low moly steel, is annealed and the threads then ground in followed by a hardening heat treatment, the threads lose their preciseness due to volume change in the work piece as it undergoes martensitic transformation. It would be desirable, therefore, if after the threads are ground in the annealed condition, the threaded surface could be hardened without adversely affecting the dimensional precision thereof.

In a composition made up of primary grains of a saturated solid solution of tungsten carbide in titanium carbide dispersed through a high alloy tool steel matrix, such as a matrix of high speed steel, it may be desirable that the heat treated composition be doubled or triple tempered at say 535 C. (about 1000 F.) to increase the toughness thereof, provided that at the same time the surface of the steel between the carbide grains is maintained at a relatively high hardness level, as might be desired for a draw die.

It is thus an object of the invention to provide as an article of manufacture a composite metal structure comprised of hard primary grains of a refractory carbide distributed through a matrix metal characterized in that at least the surface of the matrix metal between the grains is at a relatively high hardness level.

Another object is to provide a method for inhibiting the preferential wearing or erosion of a matrix metal of a composite metal structure, such as a cemented carbide composition.

These and other objects will more clearly appear when taken in conjunction with the disclosure and the appended drawing, wherein:

FIGS. 1 and 2 are representations of a photomicrograph (about 200 times magnification) of sintered compositions comprising primary carbide grains dispersed through a steel matrix showing at and below the surface thereof a diffused nitride zone between and around the carbide grains;

FIG. 3 depicts a thread gage to which the invention is applicable;

FIG. 4 represents a draw tie in which a die nib of a heat treatable refractory carbide composition is shrinkfitted into a back-up steel member; and

FIG. 5 is a fragment of the die shown in FIG. 4 indicating in an exaggerated cross section the hardened surface of the die nib.

In accordance with the invention, the preferential wearing or erosion of the matrix metal of the composite metal article containing primary carbide grains is inhibited by nitriding the exposed matrix metal surface so that the spaces between the primary grains of refractory compound material are surface hardened to anchor carbide grains in and disperse them through a nitrided layer of the matrix metal.

Examples of refractory carbide material employed in the composite includes carbides, of chromium, tungsten, molybdenum, titanium, zirconium, hafnium, niobium, tantalum and vanadium, and the like. The amount of refractory carbide employed may range from about 25% to 80% by volume. However, we find that about 35% to 70% by volume of the refractory carbide to be particularly advantageous for our purposes.

The matrix meta-1 constituting the balance may comprise any steel or alloy containing nitride-forming elements. Examples of steel compositions are: 1.25% Cr, 0.45% V, 0.3 to 0.4% C and the balance substantially iron; 1.25% A1, 1.5% Cr, 0.2% M0, 0.3% C and the balance substantially iron; 3.5% Cr, 1% V, 2.5% M0, 0.5% Mn, 0.2% Si, 0.35% C, and the balance substantially iron; 5% Co, 18% W, 4% Cr, 1% V, 0.75% C, and the balance substantially iron; and 18% W, 4% Cr, 1% V, 0.7% C and the balance substantially iron. Steels of the non-heat treatable variety may be used, such as 18% Cr, 8% Ni and the balance substantially iron, or 18% Cr, 12% Ni, the balance substantially iron and similar stainless steels. Because most of the nitride-forming elements are generally strong carbide formers, it is important that such elements dissolved in the matrix be in stoichometric excess of the carbon present in the matrix. Examples of strong carbide formers which also form nitrides are Ti, Zr, Hf, W, Cr, Mo, Nb, Ta and V. Examples 4 of nitride formers which are not strong carbide formers are Al, Mn, Si, etc.

The various refractory carbide compositions to which the invention is applicable are those generally produced by powder metallurgy. In the case of a steel bonded carbide, for example, a composition comprising primary grains of titanium carbide dispersed through a low chrome low moly steel composition, such a composition comprising 40% by weight of TiC and 60% by weight of steel may be produced as follows:

A 1000 gram charge of TiC powder of about 5 to 7 microns in size is mixed with a 1500 gram charge of a steel-forming mixture calculated to form a matrix steel containing about 1.25% Cr, 2.5% M0, 0.4% C and the balance carbonyl iron powder of about 20 microns in size. The powdered ingredients haxe mixed therewith 1 gram of paraffin wax for each grams of ingredients. The mixing is carried out in a steel mill for about 40 hours with the mill half full with steel balls, using hexane as the vehicle.

After completion of the milling, the mixture is removed and vacuum dried. A proportion of the mixed product is compressed in a die at 15 tons/ sq. in. to form a die nib of the desired size. The die nib is then liquid phase sintered at a temperature of about 1435 C. (about 2615 F.) for about one-half hour at a vacuum corresponding to about 20 microns of mercury or better. After completion of sintering, the assembly is cooled and then annealed by heating to 900 C. (about 1565 F.) for 2 hours followed by cooling at a rate of about 15 C./hour to about 100 C. (212 F.) to produce an annealed microstructure containing spheroidite. The die nib is finish machined to the desired size and is hardened to about 70R by austenitizing at about 980 C. (about 1710 F.) followed by quenching in an oil bath. The nib is then cleaned up and nitrided. One method is to subject the heat treated nib to the action of ammonia gas at a temperature ranging from about 500 to 650 C. (930 to 1200 F), for example from about 500 to 538 C. (930 to 1000 F.). As the ammonia gas decomposes to a certain extent, a very active nitrogen is formed at the moment of decomposition which combines with the nitride formers in the steel e.g., Cr, Mo, etc., to form a very hard surface on the matrix metal surrounding the primary carbide grains by virtue of the formation of nitrides in the matrix in a fine state of dispersion. It is important when the nib is heat treated prior to nitriding that enough of the surface is removed to eliminate any traces of decarburization. This is important in order to inhibit the formation of a brittle nitrided core which tends to spall. In this connection, it is advantageous to produce the die nib with a slight oversize to allow for stock removal prior to nitriding. As a result of the nitriding temperature, the core hardness of the composite steel composition drops from about 70R to a hardness within the range of about 55 to 64R Following nitriding, the nib is then shrink-fitted in a back-up annular steel member, such as depicted in FIG. 4 which shows an annular die nib 10 with a bore 13, the die nib being mounted in an annular pocket 11 of back-up steel member 12. In shrink fitting the die nib into the pocket, the back-up steel member is heated to 425 C. (about 800 F.) and the nib then inserted snugly into position in the pocket, and the heated steel then allowed to cool to room temperature.

As stated hereinbefore, since normally the matrix steel is substantially softer than the primary titanium carbide grains it will tend to selectively erode away when the die is used even though the die nib is particularly stiff and resists deformation because of its relatively high Youngs modulus which is in the neighborhood of about 44 10 p.s.i. Thus, to utilize the inherent and advantageous properties of the titanium carbide steel, we find that providing a hardened surface between and surrounding the primary carbide grains to be particularly helpful in avoiding the difficulties mentioned hereinbefore. After the die nib is shrink-mounted in the back-up member, it is lapped and ready for use.

The depth of hardness produced by gas nitriding may range from about .0005 to 0.005 inch and sometimes even higher depending on time of treatment. Referring to FIG. 5, a fragment of the die depicted in FIG. 4 is shown with the nitrided surface indicated by the numeral 14, the depth of the nitrided surface being exaggerated for illustrative purposes.

Thread gages of the type shown in FIG. 3 may be similarly treated. Thus, to avoid volumetric distortion engendered by heat treatment, the titanium carbide tool steel can be hardened in the annealed state by nitriding the threads at about 538 C. (e.g., 1000 F.) by either gas or salt nitriding. Where salt nitriding is employed, a method which may be used is as follows:

A typical commercial formulation is a liquid bath comprising a mixture of sodium and potassium salts in which the sodium salt may range from about 60 to 70% by weight of the whole mixture with the potassium salt about 30 to 40% of the whole mixture. The composition of the sodium salt consists of 96.5% NaCN, 2.5% Na CO and 0.5 KCl, while the potassium salt consists of 96% KCN, 0.6% Na C0 0.75% KCNO and 0.5% KCl. The bath nitriding temperature is about 565 C. (1050 F.).

Where a thread gage is desired which will resist a corrosive environment, the refractory carbide composite may comprise primary carbide grains of TiC dispersed by liquid phase sintering through a substantially non-heat treatable stainless steel matrix. The method employed of producing the article is similar to the method of making the di described hereinbefore. In this instance, TiC is mixed with powdered ingredients formulated to provide a stainless steel of the 18% to 20% Cr and 8% to 12% Ni variety. A rod is then produced by compacting the mixture containing 45 wt. percent TiC and 55 wt. percent steel and the rod liquid phase sintered in vacuum at a temperature of about 1435 C. (about 2615 F.) for about one-half hour in a vacuum. After completion of the sintering, the rod is furnace cooled to room temperature and has a hardness of about 47R The rod is finish machined to the desired diameter and threads 15 (note FIG. 3) precision out along one length of the rod while the opposite length 16 is knurled. Thereafter, the thread gage is nitrided to a hardness of about 58R which threads are thereafter cleaned and lapped to the final size.

As illustrative of the kind of metallographic structure which obtains when a TiC-stainless steel composition of the aforementioned type is nitrided, reference is made to FIG. 2 which shows primary carbide grains 20 of TiC dispersed through a nitrided zone 21 of matrix metal near and at the surface of the specimen. In the body of the structure beyond the nitrided zone, the primary grains are shown dispersed through the tough non-nitrided stainless steel matrix. While the gross hardness of the metal after nitriding showed an increase of over Rockwell C points, it will be appreciated that this is an average value and that actually the hardness at and just below the surface is very high. Thus, where TiC is known to have an intrinsic hardness of about 93 to 94R the nitride itself, at the surface of the matrix metal and surrounding the primary carbide grains, will also exhibit an extremely high intrinsic hardness substantially above the average of 58R mentioned hereinabove.

Another refractory carbide composition capable of being beneficially treated in the manner set forth herein is one in which the primary carbide grain comprises a saturated solid solution of a tungsten group metal carbide, e.g. WC, in TiC, the grains being dispersed through an alloy steel, such as a high speed steel. Such carbide compositions are disclosed in US. Patent No. 3,053,706. As set forth in this patent, the primary grains of the solid solution carbide are preferably at saturation, that is titanium carbide is saturated with at least one of the carbides tungsten, chromium, and molybdenum. As a further pre-requisite, the saturated solid solution carbide should be substantially at equilibrium with the matrix, and the matrix should have dissolved therein an element from the group W, Cr and Mo and suflicient carbon to assure a secondary hardening effect when the matrix is tempered at a temperature of about 565 C. (1050 F.). In achieving this secondary hardening effect, a secondary carbide is formed by reaction between, for example dissolved tungsten and carbon. At the same time the secondary hardening is effected, a toughening of the matrix obtains due to the formation of tempered martensite. In the case of a high speed steel matrix, by having tungsten, chromium and vanadium in stoichiometric excess of the carbon in the matrix, the surface of the matrix can be nitrided about and around the primary carbide grains.

In producing a composition of the foregoing type for use in forming hot swaging dies, the following constituents would be employed:

15% TiC substantially a saturated solid solution of TiCWC. 9% W in the matrix.

Actually the constituents forming the matrix calculate approximately to an l841 high speed tool steel.

In producing this composition, 1000 grams of a substantially saturated solid solution of TiCWC (30% TiC-70% WC) of about 3 microns average size is mixed with 1000 grams of steel-forming ingredients, containing about 18% tungsten, 4% chromium, 1% vanadium, 0.8% carbon and the balance carbonyl iron of about 20 micron average particle size by milling in a steel mill. The powdered ingredients contain 1 gram of parafiin wax for each grams of mix. The milling is conducted for about 40 hours, the mill being half filled with stainless steel balls. Hexane is used as a vehicle. After completion of the milling, the material is removed and vacuum dried. The mixed product is then pressed to form a die slug of the desired dimensions. The slug is then subjected to liquid phase sintering at a temperature of about 1450 C. (about 2640 F.) for about one-half hour in a vacuum of about 20 microns of mercury or better. After completion of the sintering, the slug is cooled and then annealed by heating to 915 C. (about 1675 F.) for 2 hours followed by cooling at a rate of 15/hour to 700 C. (about 1300" F.) and thereafter furnace cooled to room temperature to produce a microstructure comprising primary solid solution carbide of TiCWC distributed through a steel matrix characterized by a dispersion of econdary carbide in spheroidite form. The annealed slug is then machined to the desired shape and hardened by austenitizing at a temperature of about 1255 C. (about 2300 F.) for a time suflicient, e.g. 15 minutes at temperature, to austeriitize the matrix and dissolve the secondary carbides. The slug is then oil quenched to yield a hardness of about 72R The ferrous alloy produced in the foregoing manner is characterized by a microstructure comprising approximately 50% by weight of the saturated solid solution TiCWC as a primary carbide distributed uniformly throughout a martensitic steel matrix. The heat treated shape is then double tempered at a temperature of about 565 C. (1050 F.) in a nitriding furnace so that, during the tempering of the steel, the surface is nitrided.

A representation of a metall-ographic structure showing the extent of nitriding is depicted in FIG. 1 which shows primary grains 22 of the TiC-WC solution anchored in and dispersed through a matrix having a nitrided zone 23 near the surface and a zone 24 of tempered martensite below the surface. Smaller particles of secondary carbides 25 are also depicted.

Hardness measurements taken of the nitrided surface point up the improvement in surface characteristics that otbains in composite metal structure of the foregoing types. In the case of the WC-TiC composite alloy comprising about 50% by weight of WCTiC solid solution and the balance a high speed steel matrix containing 18% W, Cr, 2% V and 0.8% C, the hardness before nitriding 70.4 R and 87. 1 R In the nitrided condition and after lapping the surface, the hardness increases to 73.7 R or 89.7 R It will be appreciated, as stated above, that the hardness readings are not indicative of the actual hardness at the outermost surface of the matrix between the carbide grains and that actually the surface is much harder.

In the case of a composite metal carbide containing 7 about 45% by weight of TiC dispersed through a substantially non-heat treatable steel matrix of 18/8 stainless steel, the hardness before nitriding was about 47.9 R and after nitriding about 58.1 R an increase of over hardness points. As stated hereinbefore, increasing the hardness of the matrix metal surface between the carbide grains results in an improved anchoring of the primary grains in the matrix metal, whereby the metals resistance to wear, erosion and the like is assured over a prolonged period.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. A surface hardened composite metal article of manufacture consisting essentially of 25% to 80% by volume of primary grains of a hard refractory metal carbide selected from the group consisting of carbides of chromium, tungsten, molybdenum, titanium, Zirconium, hafnium, niobium, tantalum and vanadium distributed through a steel matrix metal constituting essentially the balance, said composite metal characterized by a microstructure at least adjacent the surface of said article comprising primary grains of said refractory metal carbide anchored in and dispersed through a nitrided layer of said matrix metal.

2. The composite metal article of manufacture of claim 1, wherein the refractory metal carbide is titanium carbide ranging from to by volume of the total composition.

3. The composite metal article of manufacture of claim 1, wherein the refractory metal carbide is selected from a mixture of titanium carbide and tungsten carbide ranging from 35 to 70% by volume of the total composition.

References Cited UNITED STATES PATENTS 2,393,323 1/1946 Hungerford 2919l.2 X 2,786,003 3/1957 Hollinsworth 148-16.6 2,933,386 4/1960 Pessel 14816.6 X 2,942,335 6/1960 Wellborn 29182.7 3,053,706 9/ 1962 Gregory 148-31 3,141,801 7/1964 Prutton 148-16.6 3,145,458 8/1964 Kloepfer 290182.7

HYLAND BIZOT, Primary Examiner. 

